Undersea measurement cables are used in particular for measuring temperature or pressure, e.g. at different depths under water. In general, they are carried and towed by boats or submarines. Reference is made more particularly below to a particular measurement cable called a "bathythermal" cable, but this example is not limiting.
A bathythermal cable is fitted with thermistors and is intended to measure temperature in a liquid medium, and more particularly the sea, and at depths which, at present, may be as much as about 120 meters (m).
At the surface, a bathythermal cable includes a drifting buoy connecting a main cable having a plurality of thermistors mounted thereon to an electrical device for receiving and processing information provided by the thermistors. The cable may have up to 10 thermistors. More precisely, the main cable comprises a central core constituted by a plurality of conductors and covered by a first sealing insulating sheath referred to as the "intermediate" sheath. Armoring is disposed around the intermediate sheath, e.g. in the form of steel braid or strip, serving to protect the intermediate sheath against possible puncturing and to provide the mechanical strength of the cable, said armoring being itself protected by a second sealing insulating sheath referred to as the "outer" sheath. Each of the thermistors is electrically connected to the main cable. To do this, the central core of the main cable is stripped over a portion of cable of appropriate length for making the connection. To perform this operation, all three layers of protection (outer sheath, armoring, intermediate sheath) are cut on either side of the portion to be stripped, such that the ends of the three layers of protection lie in common sections on either side thereof. All three layers of protection cut in this way are then removed over the portion to be stripped. Thereafter, one of the conductors of the core is cut in order to be bonded to the terminals of the thermistor to be connected. Each thermistor is connected in this way to a separate conductor of the central core. Sealing is provided at the connection between a thermistor and the main cable by means of locally over-molding insulating material that constitutes a sleeve extending around the thermistor and over the ends of the outer sheaths on either side of the thermistor.
The end of the bathythermal cable opposite to its end connected to the buoy has a pressure sensor. The cable may also possibly include a strength member which is disposed eccentrically or which is centered relative to the main cable (in which case the conductors of the central core are disposed around the strength member).
It is clear that like any electrical measuring apparatus that is immersed in a liquid, a bathythermal cable must be perfectly liquid-tight both at its ends and longitudinally, and above all where the over-molding is performed.
Unfortunately, in particular when used in hot seas, the outer sheath and the over-molding run the risk of being torn, e.g. by shark bites. Under other conditions, the sealing of the bathythermal cable may be broken by catching on sharp rocks or on boat anchors, in particular.
Thus, when the outer sheath is punctured, water infiltrates into the armoring, runs along the armoring since it is difficult to achieve perfect contact between the metal of the armoring and the insulating material of the outer sheath, and then reaches the thermistors by running along the connections between the over-molding and the protective sheaths. This leads to the electrical system being degraded, in particular by giving rise to short circuits, thus making measurement impossible.
Similar problems are observed at joints between two cables such as the main cable described above. The method of connecting one such cable to another is similar to that of connecting the above main cable to an electronic component: all three layers of protection are cut and removed from the ends of the cables to be connected together such that the ends of all three protective layers lie in the same section, then the cores are connected together, after which over-molding re-establishes sealing over the assembly. Here again, as in a measurement cable, given that sealing is not guaranteed in the contact zone between the metal armoring and the insulating outer sheath, water can flow therebetween until it reaches the interface between the over-molding and the end section of the three layers of protection, after which it can reach the central core and degrade the electrical connections.
The object of the present invention is thus to provide an undersea electric cable and an undersea measurement cable capable of preventing water from penetrating to their conductive cores.